MV substations play a crucial role in the power distribution network, which means that substation failures are almost always costly and disruptive. There is a good case, therefore, for regular testing to find problems before they develop into failures, but taking substation equipment out of service for testing is often difficult. Fortunately, on-line partial discharge testing offers an effective solution to this dilemma.

Partial discharge (PD) is an electrical discharge that occurs across a section of insulation between two conducting electrodes, but does not completely bridge the gap between the electrodes. PD can, for example, occur in voids in solid paper or polymer insulation, in gas bubbles in liquid insulation and, as corona, around an electrode in a gas. PD sources radiate both electromagnetic energy and acoustic energy, with the peak intensity of the acoustic radiation usually falling in the ultrasonic region.

Research and experience have shown that many types of PD are extremely damaging to the health of insulation systems. In MV substations this is an issue of considerable importance, particularly as IEEE statistics indicate that up to 90% of failures in some types of high voltage equipment are the result of the deterioration of electrical insulation because of age or stressing.

Various techniques have been developed for detecting and measuring PD, many of which require the equipment under test to be de-energised during testing. In appropriate applications, such as pre-commissioning testing of new cables, these offline techniques are invaluable, as they can provide accurate and detailed information about PD magnitude and location.
MV substations
Offline PD testing is, however, neither the most suitable nor the most convenient choice for routine “first-line” testing in MV substations. For this application, on-line PD field measurement is a much better option as it does not require equipment to be taken out of service for testing and it provides a true indication of the PD performance of the equipment with its normal operating voltage and load.
On-line PD field measurements are non-destructive and, as no over-voltages are used, the equipment under test is not exposed to stresses greater than those it experiences under normal operating conditions, which means that existing problems will not be worsened. In addition, results from tests performed at different times are directly comparable, so it is possible to trend them as a further aid to detecting insulation deterioration and to help gain an understanding of the effects of changes in environmental conditions (temperature, humidity, etc.) and service conditions.

In short, PD field measurements are ideal as a first line of defence against insulation deterioration in MV substations. The measurements can be performed easily and quickly – often in a matter of seconds – without disruption or interruption of service, and they provide results that clearly reveal where more detailed investigations are needed to guard against potential failures.

Having established the value of on-line PD field measurements, now let’s turn to the very practical aspect of how such measurements are made. Unlike a lot of test equipment for power installations, on-line PD surveying tools are typically small, light handheld instruments but in spite of their small size they offer multiple methods for detecting PD. One of today’s most useful and popular instruments, for example, offers four options – a transient earth voltage (TEV) sensor, and airborne acoustic (AA) sensor, an AA sensor with parabolic reflector and locator, and a high frequency current transformer (HFCT) sensor.

Each of the sensors covers a specific range of applications and each is used in a slightly different way. TEV sensors detect electromagnetic radiation from PD sites, which is induced into the metal enclosure of the equipment under test. The sensor is attached to the metal enclosure close to vents, seams and gaskets, and to cable terminations. This arrangement, in effect, makes it possible for the PD survey instrument to see PD sources “through” the metal enclosure.

AA sensors detect ultrasonic sound that is transmitted through the air from corona and surface discharges in air-insulated plant such as switchgear. AA sensors are placed over vents, gaps or seams in the housings of MV plant, and should be positioned so that there is a “line of sight” in air to the PD source.

AA sensors with parabolic reflectors are essentially the same as AA sensors but the addition of a parabolic reflector increases their sensing range, typically to 15 m or more. This type of sensor is particularly useful for testing outdoor overhead plant, including power transmission lines and insulators.

The last type of sensor – the HFCT – is the most suitable for detecting current impulses from PD in cables, cable terminations and the plant or switchgear in which the cables terminate. HFCT sensors are attached to the MV cable earth strap/drain wire or to the power cable with the earth strap/drain wire routed back through the sensor.

It can be seen that, with the range of sensors mentioned, first-line testing can be carried out quickly and easily on almost any type of substation equipment. The best on-line PD surveying instruments also deliver results that are easy to interpret. The SebaKMT PDS Air from Megger, for example, has a display that incorporates seven LEDs, colour coded green, yellow, orange and red, as well as a digital PD intensity readout for use with TEV sensors.

The LED display works with all types of sensor and can be interpreted at a glance. Essentially, if only the green LED is lit, the plant is healthy and in most cases will not need retesting for twelve months. If either or both of the yellow LEDs are lit, a moderate level of PD is present and more frequent retesting is recommended.

When one or both of the orange LEDs are lit, a moderate to high level of PD has been detected, and further investigations should be carried out as soon as possible to determine its source. Finally, if one or both of the red LEDs are lit, a high level of PD is present and further tests should be carried out immediately to determine and locate the cause, and to decide whether plant should be taken out of service or have access restricted.

These recommended actions are, of course, guidelines only and, in specific instances, there may be special factors that have to be taken into account when determining the meaning and implications of the on-line PD measurements. Nevertheless, the recommendations are based on long experience of testing MV plant, and are relevant in almost all cases.

On-line PD testing is, of course, not the complete solution for fault finding in MV substations, but used carefully and regularly it can provide invaluable warnings of developing problems at an early enough stage to allow remedial action to be taken before the problem progresses to become a major failure. In short, it’s an excellent, convenient and cost-effective first line of defence.

There can be no doubt, therefore, that investing in on-line PD test equipment and in regular on-line PD surveys is a very good use of money, as the potential for making savings by reducing the incidence of costly disruptive faults is enormous.